The middle part of polymethoxy dimethyl ether (CH3O(CH2O)nCH3, DMMn, n=1−8) is paraformaldehyde, and the both ends thereof are terminated by methyl groups. DMMn, wherein n is ≦5, has excellent solubility and very high permeability, is miscible with water and many organic solvents, and can be widely used in industrial solvents, pigment dispersants, or the like. DMMn is also an oxygen-containing blending component with great potential for diesel fuel, which has high cetane numbers (DMM2: 63, DMM3: 78, DMM4: 90, DMM5: 100) and high oxygen contents (42%-49%). Addition of DMMn in a amount of 10-20 v/v % to diesel fuel can improve the combustion characteristics of the diesel fuel significantly, increase the thermal efficiency effectively, and reduce the emission of NOx and carbon soot greatly. It was reported (U.S. Pat. No. 7,235,113) that the emission of the NOx, particulates and hydrocarbons in the exhaust gas can satisfy the Euro V criteria by adding 15 v/v % of DMM3-6 to diesel fuel.
A conventional process for synthesizing DMMn is by the acetalization reaction between a compound providing paraformaldehyde and a compound providing methyl groups for termination in the presence of an acidic catalyst. Since 2001, Snamprogetti S. P. A. (EP 1505049 A1, U.S. Pat. No. 6,534,685), BASF (WO 2006/045506 A1, CA 2581502 A1, US 20070260094 A1) and the like successively developed the processes for continuously preparing DMMn by the reaction between paraform or trioxymethylene and methanol, in which a liquid acid such as H2SO4 or CF3SO3H is employed as the catalyst; after the completion of the reaction, the catalyst and the by-product water were removed from the reaction liquid by adsorption with a silica gel or resin; then a two-stage rectification process is used to separate the unreacted raw materials, light components (DMM1-2), products (DMM3-5) and heavy components (DMMn≧5); and the raw materials of the reaction, the light components and the heavy components are recycled to the reactor for reuse. In the above-mentioned separation process, a great amount of adsorbents are used, the energy consumption is large, the catalyst cannot be recycled, and the cost of the raw materials is relatively high.
In 2008, BASF AG (US 20080207954 A1) investigated a process for preparing DMM1-5 by the reaction between formaldehyde and methanol in an aqueous solution, in which a liquid acid or a solid acid is used as a catalyst; the reaction liquids (DMM1-5, unreacted raw materials and water) are distilled out from the top of the rectification tower by the reaction rectification technique; then the light components (DMM1-2 and unreacted raw materials), the crude product (DMM3-4 and water) and the heavy components (DMMn>4) are separated by a multi-stage rectification process. DMM3-4 and water are separated by a phase separation technique from the crude product, and the light components and the heavy components are recycled to the reaction unit for reuse. During the practical operation, however, methanol, water and DMMn tend to form an azeotrope, so it is difficult to separate methanol from DMM3-4, and methanol has good miscibility with water together with DMM3-4, which renders the phase separation to be difficult.
In recent years, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences reported (U.S. Pat. Nos. 1,315,439, 13/164,677) a process for synthesizing DMMn by catalyzing the reaction between trioxymethylene and methanol with an ionic liquid, in which the reaction liquids were sequentially subjected to the following processes: flash distillation, thin film evaporation separation and phase separation, and thus the separation of the light components (DMM1-2, a portion of water, unreacted raw materials), the crude product DMM3-8, and the catalyst, are achieved. However, the crude product still contains a small amount of water and the catalyst, which need to be removed by adsorption with silica gel or an anion exchange resin in order to realize the refining of the product DMM3-8. In this process, the recovery rate of the catalyst is relatively low, the cost of the raw materials of the reaction is high, and the process flow is long.